Depending upon the size of the speakers, weight and overall dimension of each shipping box, you may or may not be able to utilize UPS Ground with which I've had good luck shipping heavy amplifiers (120 pounds each), high-end transport, DAC and clock, etc....
UPS does have a dimensional weight restriction of 150 pounds per box so it is likely you'll need to look at other options.
I have very large floor-standing speakers (Legacy Audio Focus 20/20 and Focus HDs). These weigh in at 205 pounds each and their shipping boxes are about the size of a refrigerator. I've had very good luck and no damage over the past 3 years with multiple speaker shipments of this size, weight and dimension as well as smaller speakers (50-90 lbs) using Yellow Freight. This is also Legacy's preferred/only carrier. The beneficial aspect of shipping with Yellow (or other LTL truck carrier) is that they come pick up the shipping crates, strap them and shrink-wrap them to a pallet and know how to handle the weight and size of these items. There are few things to remember in that you must allocate time to arrange for pickup, be home when they are to come for the pickup and ensure they pallet everything to your satisfaction. You should also remember that when you sign the BOL (Bill of Lading) at the origin, and the receiver signs at the destination, you are stating that the shipment is in good order. The receiver should not sign the BOL until after he/she inspects the shipment in the driver's presence. Like any delivery with any carrier, after you accept the shipment and the driver leaves, it's very difficult to log any complaint of damage after the fact.
That stated, this is an option that has worked for me several times in the last 3 years. There are other
carriers (UPS Freight, FedEx Freight/LTL, etc...) but I have not used them.